This invention relates to a communications system capable of transmitting and receiving information in encrypted form to and from other communications apparatus such as a telephone, radio communications equipment or facsimile machine.
In such a communications system, it is desirable to assign a unique cipher key to each of other communicating user terminals. If, however, the number of user terminals increases, the amount of cipher key data becomes so large that it becomes difficult to manage such data. One known arrangement for facilitating management of cipher key data is to assign a common cipher key to a plurality of other user terminals.
As a known example of communications systems utilizing such an arrangement, one form of facsimile machine is provided with a "cipher key table" in which a plurality of cipher keys and their index numbers are updatably registered as well as a "user terminal table" in which individual users' facsimile numbers, cipher key index numbers, etc. are updatably registered. With this arrangement, cryptographic communications can be automatically performed by using a specified cipher key.
In this conventional arrangement, cipher keys are first registered with reference to index numbers in the cipher key table, and then the index numbers are registered for individual users in the user terminal table. One problem of this conventional arrangement is its operational inconvenience in registering data in only one of the two data tables.
Furthermore, if alteration or deletion of data in the cipher key table and user terminal table is required, an index number in the user terminal table must be altered or deleted at first, and then the corresponding cipher key in the cipher key table must be altered or deleted. This is another operability problem of the conventional arrangement.
It would be possible to reconfigure the above-described conventional arrangement so that data be separately registered in the cipher key table and user terminal table. In this modified form of the conventional arrangement, however, it may happen that although a cipher key index number is registered in the user terminal table, the corresponding cipher key is not registered in the cipher key table. If an instruction is given to perform cryptographic communications in this situation, a communication error occurs resulting in an inability to communicate.